U.S. Pat. No. 3,342,236 shows various means for increasing the holding power of a free spinning bolt or cap screw, one of which comprises a double size groove on the bearing face of the bolt head. I have discovered that a double size groove may also be used to increase the holding power of a free spinning nut in applications where the nut is made of harder or denser material than the surface against which it is forced and that the holding power and other advantages may be improved or obtained by fluting or weakening the side of the bolt, preferably in alignment with the larger part of the groove.